Secret signaling system



'Dec. 6, 1949 'c. E. ATKINS SECRET SIGNALING SYSTEM s Sheets-Sheet 1 Filed Sept. 8, 1945 1N VENTOR. CA R1. 5 0 WARD A TK/NS ME 3 33 M49 0 Db w m8. 3 Some 8 w h g vod wn five V mmE r E 7 32 E EGG A .32 E uwm V N% 8mm Q halts k w m m- A l Gk bzgwfi w 3 FEE 02m: omkk 6 m V QQN ATTORNE Y5 Dec. 1949 c. E. ATKINS SECRET SIGNALING SYSTEM 3 Sheets-Sheet 2 Filed Sept. 8, 1945 INVENTOR. CARL 'EDWA/PD ATKINS 6 M [0% BY awn;

ATTORNEYS mew-I C. E. ATKINS 3 Sheets-Sheet 3 WWI-- INVENTOR. CARL E owmo Arm/vs SECRET SIGNALING SYSTEM Dec. 6, 1949 Filed Sept. s, 1943 zm M/uw ma ATTORNE Y S Patented Dec. 6, 1949 UNITED STATES PATENT OFFICE SECRET SIGNALING SYSTEM Carl Edward Atkins, Evanston, .Ill., assignor..t0 Tung-.-.Sol Lamp Works, Inc.,.Newark, .N. .J., a corporation of Delaware g Application September 8,.1943, SerialNo. 501,529

"of two stations with means at each station to scramble the-outgoing -signal with the signal introduced at the other station; the signal energy,

or amplitude or frequency modulation corresponding thereto, introduced at each station making a completeloop and returning to the stationof origin before elimination from the system. The system of the present invention relates more particularly to the type of secret signaling system disclosed and claimed in Serial No. 468,345, in which theequipment at the two stations, together with the intervening'space, form an endto-end oscillator and frequency modulation is employed in the transmission of intelligence.

The principal object of the present invention is in general to provide an improved and simplified secret signaling system of "the above .mentioned type and an irnproved methodof operation .thereof. Another object :of the invention is to provide in such a system means for increasing the 'diiiiculty of unauthorized interception of the communicated intelligence.

Still another object of the invention is to pro- I vide, in such a system, means for preventing an interloper from taking an accurate bearing on either or both of the communicating stations.

Other "objects and features of 'the invention will become apparent as the description proceeds. I;

For an understanding of the invention, reference-may be had to theaccompanying drawings,

of which- Figs. 1 and 2 are diagrammatic circuit drawings of the apparatus at two communicating stations equipped with a secret signaling'system embodying the invention;

.Fig. 3 is a diagram explanatory of the operation of the system of Figs. '1 and 2; and

Fig. dis a circuit diagram of apparatus at either =1 of two communicating stations adapted for use in accordance with :the invention when secrecy of the location of the station is desired.

The invention asrembodied in the disclosure of Figs. 1 and 2 will first be described with reference a each station there isa receiving antenna 2, freand A quency divider 4, tuned-circuit 6, limiter 8, frequencymultiplier andtranSmitter Ill, and transmitting antenna l2 connected in a chain in the order-stated and these twochains of equipment, together with the intervening space between the stations constitute an end-to-end oscillator the frequency of which varies from point to point but is fundamentally determined by the circuits to and 6b. As transmission and reception at each station is preferably at substantially different frequenciesthe constants of dividers 4a and 42) will differ, as .willzzthose .of the multipliers Illa lilo. If, for example, circuits 6 are arranged to operate .at 500.kc. and unit No multiplies this frequency. by .five, thus applying 2.5 me. to transmitting :antenna 12a, then unit 41) will .divide .the received frequency by five, yielding :500 kc. in circuit .6b,.and unit lllb may multiply by three, transmitting 1.5 mc. to station A where .unit 40!. will divide by three to return the frequency to500 kc. in circuit 6a. Units 4 are preferablyof the cascaded variety and have a number of stages, depending upon the number of divisions necessary in the particular design. These units :should also provide radio frequency amplification and suitable selectivity as will be apparent to those skilled in the art.

A reactance tube [4 at each station is connected in shunt with the tuned circuit 6 thereof. The bias on each tube I4 is shifted in response to audio frequency voltages appearing across a resistor l6; an adjustable tap [1 0n resistor I5 being connected to the .tube 14. With this arrangement the system may be simultaneously frequency modulated at the two stations without cross modulation, provided the frequency excursions are kept within reasonable .limits.

Signal detection is achieved by feeding the signal after it passes through limiter 8 to an amplifier stage I3 in the output of which is a frequency detecting network which is preferably that known as the Seeley circuit. As shown, such circuit includesa pair of circuits .20 and 22 both tuned to the frequency .of circuit 6 under idling conditions and inductively and conductively coupled together. The circuit includes also a pair of diodes 24 and a pair of series connected resistors 26 cooperating therewith so that audio frequency voltages appearing across the resistors ZE-and corresponding to the-frequency deviations of the system may bedelivered through a lead 28 to any suitable audio frequency responsive device such as a loud speaker or the like (not shown).

With the-circuitso.far described, the detector v to key clicks. relevant clicks it would be-difiicult if not impos- 3 of station A, for example, will respond to all frequency modulated signals applied to the input of amplifier 8a whether such modulation originated at station B or station A. In order, therefore, to suppress, in the detecting circuit the locally introduced modulation, the circuit 22 is connected through a small blocking condenser 36 with a reactance tube 32 which is excited in suitable phase by the same audio frequency energy that actuates tube M. For this purpose tube 32 is connected to .a tap 33 on a resistor 34 connected in parallel with resistor 16. If the taps I1 and 33 are properly adjusted and the characteristic curves of tubes l4 and 32 are substantially identical and the modulation free from distortion,

complete cancellation of the locally introduced energy is efiected in the detecting circuit. With the above described arrangement tubes Ma and Mb cause simultaneous frequency modulation of the system in response to audio frequency energy I introduced at stations A and B respectively while the audio frequency output voltages appearing at 280. correspond only to those introduced at station B and the audio frequency output voltages appearing at 28b correspond only to those introduced at station A.

With the system so far described, straightforward simultaneous modulation results and an eavesdropper will hear both messages at once.

The masking of one signal by the other, particul arly if some simple but different system were employed at each station for scrambling the audio frequency energy before application to the reactance tubes, makes thedecoding of the transmitted signals by an interloper an exceedingly difiicult procedure. To insure substantially complete secrecy I prefer not to rely only upon the masking of one signal by the other, whether preliminarily scrambled or not, but to employ the method and apparatus now to be described. At

each station a, low frequency monotone of say 400 c. is keyed telegraphically and injected as modulation. This modulation injected at station A will appear at station B with the same depth of modulation and with a definite phase angle.

The tone modulation injected at station B may increase or decrease the modulation depth of the original modulation, depending upon the relative phase angle of the injections. If, however, the phase angle between the two injections is exactly I 120, then there is no increase or decrease in the overall amplitude or depth of modulation. Such a condition is shown vectorially in Fig. 3, wherein 36a represents the tone modulation introduced at station A, 36b that added at station B, and 38 the resultant modulation, being a change in angle only and not in magnitude. I have found that by using this vector relationship in applying the keyed modulation at the two stations that there is no change in the depth of modulation regardless of whether both keys are down or only the key at A or only the key at B is down. By insuring that there is no appreciable modulation departures or that it is the same at the two points there is no evidence of simultaneous key depression. There is of course a change of phase of the modulating signal whenever a second key is depressed when modulation is already in progress.

, This .change in phase would appear to an enemy as a key click but since irrelevant key clicks may be introduced without interfering with the intelligibility to authorized persons I believe it would be impossible to decipher the coding by listening Even Without the injection of irsible to ascertain whether a given click represented the end or beginning of a given assignable symbol.

In practice two messages could be sent simultaneously in the opposite directions, the one telegraphic message serving to completely mask the other. If this is not desirable an irrelevant series of clicks may be sent in one direction or arrangements could be made to record the signals going from A to B and then send them back in an inverted form or in a different phase relationship in order to provide adequate masking when a telegraph key was operated from only one end.

Suitable apparatus for injecting this modulating tone at the two stations is indicated in Figs. 1 and 2 and comprises, at station A, a pilot carrier "oscillator 40 connected to the transmitting antenna Ma and which is modulated by the output from a 400 cycle oscillator 42 supplying the modulating tone. The 400 cycle energy of oscillator 42 is delivered to the resistors Mia and 34a under control of the key 44a. At station B the apparatus includes a receiver 46 connected to the receiving antenna 2b, a 400 cycle oscillator 42b which is locked by the output energy from the receiver 43, an adjustable phase shift network 48 for insuring the proper 120 angle between the modulating tones and the key 44b controlling the introduction of audio frequency energy from the oscillator 40b to the resistors 5b and 34b. The operation of the above described apparatus for injecting the low frequency modulating tone at each station with a difierence in phase angles of 120 will be clear from the preceding discussion. Other apparatus suitable for the purpose will occur to those skilled in the art.

The entire secret signaling system as illustrated in Figs. 1 and 2 and as above described differs from systems disclosed and claimed in my prior applications not only in the inclusion of the means for injecting the modulating tones but also in the provision of frequency dividers and frequency multipliers, instead of frequency convert- *signal to noise ratio, substantially simplifies the system, and by reducing the number of oscillators in the system greatly reduces the possibility of tweets and spurious responses.

The system of my present invention includes also a switching arrangement, shown in Fig. 4, by means of which enemy compass stations may be confused as to the location of the communicating stations. The apparatus of Fig. 4, hereinafter described, operates to switch the operating frequency of the communicating stations back and forth at an irregular rate. In other words, station A transmits a portion of its intelligence at a frequency is while station B is completing the end-to-end circuit on a different frequency fb. At irregular intervals station A will switch over to frequency is for transmission while station B synchronously switches tothe frequency fa to transmit the return signal. Thus if a compass station were tuned to either communicating frequency, it would be compelled to take cognizance of energy arriving from different directions and since it would be difficult, if not impossible, to synchronize the compass station with the switch-over period, especially when the latter is irregular, the only bearing that could be taken would be that indicating a vectorial addition of the incoming waves. This would result, unless the enemy compass station were very close to one :or the 1 other :of i the communicating stations,

in a drastically erroneous bearing.

Fig.4 represents.diagrammatically:the switching :arrangement :and signalin equipment at eitherzstation A or station B. In Fig. .4 the receiving antenna 2 is arranged to feed three radio frequency .transformersbfl, 52 and 25%. Trans- .Iformer -59.feeds frequency divider 59 which in- .acludes, 'aszinithe case of dividers 11a and 4b, suitenergy.introducedrat ti; is impressed upon are- .actance .tube 661'and aadetector unit 518; reactance tube 166 .and unit 3368 .each being connected in .shunt with the circuit t2. :system concerned with'the introduction of signal energy andawith'the suppression in the detector'ofzthewlocally introduced signal is intended to be :thesameas that heretofore described in-connection-withFigs. 1 and 2, it will be understood-that unit '68 of Fig. 4 includes the necessaryelements therefon'as, for example, a reacatance tube such as tube 32,-an amplifier such as amplifier 18 and the detecting circuit of Figs. 1 and2. For simplicityandbecause it is not es- :sentialito the operation-of the system, nolimiter has been shown :between the detector 63 and circuit '62but one couldb'eemployed if desired.

One frequency @multiplier and transmitter 10 .is arrangedto energizetransmitting antenna I2 through transformer 12. A second frequency multiplier and transmitter 14 energizes antenna f2 through a transformer '19. Unit 1-4 differs .from unit 10 inthat the number of frequency :multiplications is different. If, for "example, circuits 62a and -62b operate at 500 -kc., and the transmission=frequencyfa is five'times-this value,

or 2.5 mc., and the transmission "frequency is .is three times this value, or 1.5 mc., then unit 10a would multiply by five and unit 14a would .multiply'by three. =Cooperatively, unit 56a would :divide by three andjunit 58a would divide 'by five.

Similar relations of course exist at sta- -.tion B.

In practice, separate units 58 and 14 would not be required as they could be combined respectively .withunitsit and T0 and the change over to a lesser or greater number of divisions -or multiplications be effected by switching in -:or :outvariouszstages .in the combined devices.

Associated with resonant circuit 62 are electron tubes 18, 80 and 82, 84, .which serve to transpose ithe :terminals ;of circuit 62 and its asso- :ciated :controlling :and :responding units T56 and :Gflfrom units.ifiand'z'lfllto'unitsfiii and T4. These tubes rare of "the multi-grid'variety containing at least :two :control electrodes.

:One control electrode .of :tube 18 "is connected to unit '55; .one :controlelectro'de oftube :80 is connectedto unit 58; and one control grid of each of tubes "82 and 84 :is :connected to :the tank circuit 62. The plate'of tube 82 connects with unit iii, that of tube"84 withunit l4 and the plates oftubes l8 and 8.9 .areparallelconnected to the circuit :62. A :resistor iflfi 'groundedzatits .center, receives As the part of the pulsating energy froma unit 88, labeled pulse actuator. One end of this resistor 86 is connected to a control grid of each of tubes I8 and 82 while the other end of the resistor 86 is connected to a control grid of each of tubes 89 and 84. Thus when there is a substantial potential across resistor 86, one pair of grids are biased completely to cut off with a negative potential while the corresponding pair will be substantially positive, permitting such tubes to draw plate current and be operative.

Unit 90 is a pulse generator applying modulation to a transmitter 92 which feeds energy through a transformer 94 to antenna I2. Unit 90 may be any device for generating energy at intervals and preferably at erratic intervals. For example, unit 99 might be a source of random noise such as a high gain radio frequency pentode having a relatively high resistance in its control grid circuit and a filter in its output circuit passing energy of audio frequencies. Preferably the duration of the generated pulses and of the intervals therebetween are commensurate but not necessarily equal. A switch 9| is provided in the circuit connection between units 90 and 92. As shown, pulse actuator 88 is connected between receiver 60 and transmitter 92, a switch 93 being provided in the circuit connections between units '88- and 92.

In the following description, units at station A will be identified by the subscript a and those of station B by the subscript b. In operation but one pulse generator is in operation at any one time. Assuming that the pulse generator of station A is in operation, then switch Sia is closed, switch 93a is open, switch 9lb is open and switch 93b is closed. The pulses generated in unit 98a are transmitted by antenna [2a, picked up by antenna 2b and fed through transformer 54b to receiver 69b. After demodulation in receiver 59, the pulse is received in unit 88b. The output of unit 382) appearing across resistor 88b serves to switch over the receiving function from unit 5612 to unit 581) and-the transmitting function from unit 19b to unit Mb for the duration of the pulse and synchronously with such shift at station A. This synchronization is obtained by virtue of the fact that the demodulated pulse from receiver Eiflb passes through unit 881) to transmitter 92b and thence to antenna l2b. As unit 99b is disconnected by switch 9H), the pulse reaching antenna I222 through transformer 94b is the pulse originating at station A. After a suitable lapse of time this pulse is received at antenna 2a, passes through transformer 55a to receiver 58a and through this unit to pulse actuator mechanism 83a. If the timing of the pulse transmission through its transmitting and receiving devices coincides precisely with the transmission of the normal communication signal through its respective transmission channels, than the switch-over will be synchronous atzthe two stations; that is, the change-over will occur just enough later at the second point :to'allow .for the transmission of the signal on the new frequency. In the intervals between pulses tubes 73 and 82 become operative to switch units 56 and 79 back into the osciliatory chain synchronously at the two stations, that is, at times coordinated to the time required for transmission of the signal from one station to the other.

Thus, with the above described arrangement, the continual interchange of transmission frequency between the two stations will cause any erably would be included in the system of Fig. 4.

The following is claimed:

1. A secret station-to-station communication system comprising in combination equipment at each of two stations including receving means, a frequency divider, a frequency determining network, a frequency multiplier and transmitting means connected in a chain in the order stated,

said equipments at the two stations together with the intervening space forming an end-to-end oscillator, means at each station responsive to audio frequency signal energy for varying the frequency passed by said network, a detector at each station connected to said network, and means for rendering said detector unresponsive to such changes in frequency of the energy passed by the network as are due to the frequency varying means at the same station.

2. A secret station-to-station communication system comprising in combination equipment at each of two stations including receiving means, a frequency divider, a frequency determining network, a frequency multiplier and transmitting i means connected in a chain in the order stated, said equipments at the two stations together with the intervening space forming an end-to-end oscillator, means at each station responsive to audio frequency signal energy for varying the frequency passed by said network, a detector at each station connected to said network, means for rendering said detector unresponsive to such changes in frequency of the energy passed by the network as are due to the frequency varying means at the same station, a second frequency divider and a second frequency multiplier in the equipment at each station, switching means and circuit connections at each station for simultaneously replacing in the oscillatory chain the first mentioned divider at a station by said mentioned divider and the first mentioned multiplier at that station by said second mentioned multiplier and cooperating means at the two stations for synchronously operating said switching means whereby the transmitting frequencies of the two stations may be intermittently interchanged.

3. In a secret station-to-station communication system of the type wherein equipment at each station, including receiving and transmitting means and a chain of units connecting such means, form together with the intervening space an end-to-end oscillator, the combination com-- prising a frequency determining circuit in the chain of units at each station, means at each station for varying the tunin of said circuit, a low frequency oscillator and a key at each station for delivering at will audio frequency control energy to said last mentioned means, a phase shift network at one station connected between said low frequency oscillator and said last mentioned means and cooperating means at said two stations for locking the frequency of said oscillator at one station to that of said oscillator at the other station.

.4. In a secret station-to-station communication system of the type wherein equipment at each station, including receiving and transmitting means and a chain of units connecting such means, form together with the intervening space an end-to-end oscillator, the combination comprising a frequency determining circuit in the chain of units at each station, means at each station for varying the tuning of said circuit, a low frequency oscillator and a key at each station for delivering at will audio frequency control energy to said last mentioned means, a phase shift network at one station connected between said low frequency oscillator and said last mentioned means and cooperating means at said two stations for locking the frequency of said oscillator at one station to that of said oscillator at the other station, said cooperating means at said two stations including a high frequency oscillator at one station connected to the low frequency oscillator and to the transmitting means at that station for providing a carrier for the energy from the low frequency oscillator, and a demodulating device at the other station connected between the receiving means and the low frequency oscillator at that other station for locking the frequency of said low frequency oscillator.

5. The method of operating a secret station-tostation signaling system of the type wherein cooperating equipment at the communicating stations, together with the intervening space, form an end-to-end oscillator which comprises utilizing a low frequency monotone at one station as the audio frequency signal energy for controlling the introduction of frequency modulation into the system at that station and utilizing a monotone of the same frequency and 120 degrees out of phase therewith as the audio frequency signal energy for controlling the introduction of frequency modulation into the system at the other station.

6. In a secret station-to-station signaling system, the combination comprising at each station receiving means, a pair of frequency dividers each coupled to said receiving means, a frequency determining circuit, a pair of frequency multipliers, transmitting means coupled to each of said multipliers and thermionic switching means between said dividers and said circuit and between said circuit and said multipliers, operative to interpose said circuit either between one divider and multiplier or between the other divider and multiplier, means at each station for varying the tuning of said circuit in response to audio frequency signal energy, detecting means at each station responsive to changes in the frequency passed by said circuit due to variations of tuning of the circuit at the other station and cooperating means at the two stations for operating said switching means to transpose the transmitting and receiving frequencies of the two stations in synchronism.

'7. In a secret station-to-station signaling system, the combination comprising at each station receiving means, a pair of frequency dividers each coupled to said receiving means, a frequency determining circuit, a pair of frequency multipliers, transmitting means coupled to each of said multipliers and thermionic switching means between said dividers and said circuit and between said circuit and said multipliers, operative to interpose said circuit either between one divider and multiplier or between the other divider and multiplier, means at each station for varying the tuning of said circuit in response to audio frequency signal energy, detecting means at each station responsive to changes in the frequency passed by said circuit due to variations of tuning of the circuit at the other station and cooperating means at the two stations for operating said switching means to transpose the transmitting and receiving frequencies of the two stations in synchronism, said thermionic switching means including a pair of multi-electrode tubes having their plates parallel connected to said circuit and each having one control grid connected to one of said dividers, and a second pair of multi-electrode tubes each having its plate connected to one of said multipliers, said ncircuit being connected to one control grid of each of said last mentioned pair of tubes, and wherein said cooperating means include a center grounded impedance having one end connected to a second control grid of one tube of each pair and its other end connected to a second control grid of the other tube of each pair, means including a pulse generator and pulse actuator for impressing potentials across said impedance and means for transmitting the generated pulses from one station to the other for synchronous operation of said switches.

8. The method of operating a secret station to station communication system which comprises using different transmitting frequencies at the two stations, receiving at each station the signals transmitted by the other station, changing the transmitting and receiving frequencies at one station to that of the other station, synchronously changing the transmitting and receiving frequencies at the other station to the previous transmitting and receiving frequencies of the first station, and subsequently at irregular intervals similarly synchronously interchanging the transmitting and receiving frequencies at the two stations.

9. In a communication system having communicating stations, the method of obtaining secrecy in the transmission of intelligence from each station comprising modulating the signal sent from one station with a control signal for the transmission of intelligence to the other station, locking in a modulating signal at the other station with the first signal and shifting the frequency of the signal sent from said other station between two values in accordance with the adjusted locked in signal for the transmission of intelligence to the first station.

10. In a communication system having at least two stations each including at least two frequency dividers and associated transmitters, means for alternately operating each divider and its associated transmitter comprising a thermionic vacuum tube with at least two control grids for each divider and for each transmitter, connections from each divider to one of the grids of its associated tube and an output circuit connected with the plate thereof, connections from the plate of each of the other tubes to their associated transmitters and an input circuit connected with one of the control grids of each of the last mentioned tubes, connections between the remaining grid of each divider tube and the remaining grid of the companion transmitter tube, and means including a source of energy connected with the remaining grids aforesaid for alternately bringing into operation each pair of companion tubes.

11. A communication system having at least two stations comprising two transmitter receiver combinations at each station for operation at different frequencies, electronic means including a source of varying energy for alternately bringing into operation one receiver transmitter combination or the other receiver transmitter combination at one of said stations, means for transmitting said varying energy to the other station and electronic means at said other station responsive to said varying energy to alternately bring into operation the corresponding receiver transmitter combination at said other station in synchronism with the first station to provide uninterrupted communication between the two stations.

12. In an electronic circuit, means for selectively demodulating carrier waves carrying two or more signals comprising a frequency modulation detector responsive to reproduce said signals and reject the carrier wave, and including at least one tuned circuit, and means for varying the tuning of said tuned circuit in accordance with one of said signals in such phase relationship to effect cancellation of that one signal in said detector to suppress said one signal while reproducing the other signals.

13. The method of operating a secret stationto-station signaling system wherein the communicating stations form an end-to-end oscillator which comprises transmitting a monotone signal at one station of controlled amplitude to intermittently modulate the carrier for the transmission of intelligence to the other station, transmitting a monotone signal at the other station synchronized with and of an amplitude equivalent to the first signal for the transmission of intelligence to the first station and controlling the phase of the second monotone signal substantially 120 degrees relative to the first signal whereby the degree of modulation is substantially constant with simultaneous transmission of intelligence from each station to the other.

CARL EDWARD ATKINS.

REFERENCES CITED The following references are of record in the file of this patent:

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